A transmission part of radio communication equipment such as a mobile phone and a wireless LAN needs to operate with low power consumption while ensuring the accuracy of a transmission signal regardless of the amount of output power. Particularly, since a power amplifier at a last stage of the transmission part of the radio communication equipment occupies 50% or more of entire power consumption of the radio communication equipment, high power efficiency is required.
In recent years, as a power amplifier expected to have high power efficiency, a switching amplifier has been spotlighted. The switching amplifier assumes a pulse shape signal as an input signal and can amplify power while keeping a waveform of the input signal. The pulse shape signal amplified by the switching amplifier is radiated to the air by an antenna after a frequency component, other than a desired frequency component, is sufficiently suppressed by a filter element.
FIG. 7 is a circuit diagram illustrating a class D amplifier which is a representative example of a conventional switching amplifier. The class D amplifier illustrated in FIG. 7 has a configuration in which two switch elements 71 and 72 have been inserted in series between a power supply 73 and a ground (GND) 74. Complementary pulse signals S1 and S2 are respectively input to the two switch elements 71 and 72 as an open/close control signal S so that only one of the two switch elements 71 and 72 is controlled to be in an ON state. For the output of the class D amplifier, when the switch element 71 of the power supply 73 side is ON and the switch element 72 of the ground 74 side is OFF, a voltage equal to a power supply voltage is output. In a reverse case, a ground potential is output.
Since the class D amplifier requires no bias current, ideally, power loss is ‘0’ and power efficiency is 100%. The switch elements used in the two switch elements 71 and 72 can be individually configured with a MOS field effect transistor, a bipolar transistor, or the like.
FIG. 8 is a block configuration diagram illustrating an entire configuration example of a conventional radio transmitter using the class D amplifier illustrated in FIG. 7 (for example, see NPL1 and NPL2). As illustrated in FIG. 8, the conventional radio transmitter is configured with an RF signal generator 81, a driver amplifier 82, a class D amplifier 83, and the like. For example, in the case of W-CDMA, a radio signal is generated as a multibit signal of 10 bits or more in a digital baseband ‘8’.
On the other hand, an input signal of the class D amplifier 83 including the two switch elements 71 and 72 has a complementary pulse waveform as illustrated in FIG. 7, and a signal of only 1 bit having the pulse waveform can be transmitted. Accordingly, an output signal from the digital baseband 810 needs to be converted into 1 bit in advance. In the configuration example of FIG. 8, as a means for a 1-bit conversion, delta-sigma modulators 811 and 812 are used in order to maintain good noise characteristics in the vicinity of a frequency band of a desired wave. According to the present configuration, a radio signal can be converted to a pulse shape signal while maintaining good noise characteristics and can be input to the class D amplifier 83.